Simultaneous Measurement of X-Ray Absorption Spectra and Kinetics: A Fixed-Bed, Plug-Flow Operando Reactor

Introduction

X-ray absorption spectroscopy (XAS) is a powerful tool that can be used to characterize catalysts at reaction conditions. Extended x-ray absorption fine structure (EXAFS) yields information on coordination numbers, inter atomic distances and types of neighbors. X-ray absorption edge spectroscopy (XANES) reveals information on the oxidation state, coordination symmetry and, in many cases, the coverage of adsorbates. Because the structure-property of a catalyst is most important at reaction conditions, we have developed a novel operando reactor system which allows us to perform high quality XAS while simultaneously obtaining kinetic data [1]. Clausen and Topsøe [2] utilized a capillary tube as an operando XAS reactor while Bare et al. [3] have demonstrated operando XAS using a Be plug flow reactor (PFR). However, catalysts are difficult to load into capillary tubes and Be is expensive which makes these reactor designs non-ideal.

Here we describe an operando reactor capable of probing catalysts at temperatures up to 550 °C and 40 atm. We used a readily available borosilicate glass tube reactor loaded with a 2% Pd, 13.7% Zn on Al₂O₃ catalyst to examine the water-gas shift (WGS) reaction at catalytically relevant reaction conditions. In addition, we show this novel reactor is capable of determining surface coverages of H₂O, CO and H₂ under WGS reaction conditions on alumina supported Pt and Au catalysts from XANES spectra. These data correlate well to data we find with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).

Experimental

X-ray absorption measurements were made on the insertion device (ID) beamline, 10-ID-B, of the Materials Research Collaborative Access Team (MRCAT) at the Advanced Photon Source (APS), Argonne National Laboratory. The 2% Pd, 13.7% Zn on Al₂O₃ WGS catalyst was the same as Bollmann et al. [4]. The Pt/Al₂O₃ catalyst was fabricated by using atomic layer deposition (ALD): 1 cycle of Pt(MeCp)Me₃ followed by O₂ over spherical alumina (NanoDur). The 0.9% Au/Al₂O₃ was purchased from AuTEK. Two types of reactors were used: (i) a borosilicate glass tube reactor (OD: 0.25”, ID: 0.152”, 10 cm long) for the 2% Pd, 13.7% Zn on Al₂O₃ and ALD Pt/Al₂O₃ catalysts and (ii) a standard NMR tube reactor (Wilmad-LabGlass, part #: 6.5-PP-9) for the Au/Al₂O₃ catalyst. The operando reactor was heated by a custom designed, temperature controlled Al heating block. The block was outfitted with a through hole allowing access to the catalyst bed by the x-rays. The reactant gas flow rate was a standard WGS gas composition of 21.4% H₂, 6.4% CO, 11% H₂O, and balance Ar. The rate of CO consumption was used to calculate WGS rate and the effluent gas was analyzed with a gas chromatograph. Adsorption experiments of CO, H₂ and H₂O were performed on two different catalysts, ALD Pt/ Al₂O₃ and 0.9% Au/g-Al₂O₃, using the operando reactor and a DRIFTS cell (CO only). The standard WGS gas composition was used to investigate the amount of adsorption on the metal surface. The DRIFTS experiments were performed in an in-situ cell described elsewhere [4].

Results and Discussion

            To validate the borosilicate glass tube reactor as a true PFR, it was loaded with a 2.0% Pd, 13.7% Zn on γ-alumina catalyst and tested under standard WGS conditions and compared to Bollmann et al. [4]. The apparent activation energy E_a within the temperature range of 200-300 °C was 65.4 ± 1.7 kJ/mol, in agreement with 69 ± 3 kJ/mol found by Bollmann et al. [4], and the rate at 250 °C was 1.2x10^-6 ± 0.1x10⁶ mol s^-1 g-cat^-1. The rate reported from Bollmann et al. adjusted to our conditions is 1.4x10^-6 mol s^-1 g-cat^-1, a difference of 15%. At RT, the PdZn has not been fully reduced, but at higher temperatures, 250 °C to 330 °C, the catalyst shows no signs of oxidation, as evident in both XANES and EXAFS, and a Pd-Zn alloy forms. The quality of XANES and EXAFS spectra are excellent and are comparable to spectra seen elsewhere [4]. The Pd-Pd peak at 2.74 Å, Pd-Zn at 2.52 Å and Pd-O at 2.00 Å are clearly visible in the EXAFS.  

Adsorption experiments show that under WGS conditions CO covers approximately 80% of the Pt surface at 250 °C and 16% of the Au surface at 120 °C using ALD Pt/Al₂O₃ and Au/Al₂O₃, respectively. These findings match well with what we see using DRIFTS; CO covers significantly more of the Pt surface than it does Au. At higher CO surface coverages, as seen on ALD Pt/Al₂O₃, one would expect to see a lower CO order of reaction than on lower CO coverages, as seen on the Au/Al₂O₃. Indeed, this is the case; the CO order of reaction is 0.05 and 0.78 for Pt/Al₂O₃ and Au/Al₂O₃, respectively. Moreover, the amount of adsorbed H₂ seen using XANES is minimal on both the Pt and Au catalysts under reaction conditions.

Conclusion

            It has been shown that the operando XAS reactor is capable of simultaneously producing high quality EXAFS, XANES and kinetic data. These types of data aide in adsorbate and structure-property characterizations at relevant reaction conditions, liquid or gas phase. These types of experiments are made possible by high flux 3^rd generation ID beamlines.

Acknowledgements

The authors would like to thank the Advanced Photon Source (APS) at Argonne National Laboratory and the Materials Research Collaborative Access Team (MRCAT) at beamline 10-ID-B.

References

1)         Fingland, B.R., Ribeiro, F.H., and Miller, J.T., Catalysis Letters (2009) Accepted for publication.

2)         Clausen, B.S., and Topsøe, H., Catalysis Today 9 (1991) 189-196.

3)         Bare, S.R., Yang, N., Kelly, S.D., Mickelson, G.E., and Modica, F.S., Catalysis Today 126 (2007) 18-26.

4)         Bollmann, L., Ratts, J.L., Joshi, A.M., Williams, W.D., Pazmino, J., Joshi, Y.V., Miller, J.T., Kropf, A.J., Delgass, W.N., and Ribeiro, F.H., Journal of Catalysis 257 (2008) 43-54.